This invention relates generally to a process for manufacturing a semiconductor wafer, and more specifically it relates to a method and apparatus for placing a mark on a wafer to identify it during the manufacturing process.
In the manufacture of semiconductor chips, a wafer of silicon or other semiconductor material is processed to form the individual chips. In a process step that is relevant to this invention, a layer of photoresist is spread over the surface of the wafer and is exposed and developed to produce a pattern of openings in the resist layer. The openings expose the underlying structure for the next process step and the resist areas that remain after developing isolate the structure during this step. Commonly, the resist forms a mask for etching the wafer.
Individual wafers are identified during manufacture. For example, several wafers can be identified by a common lot identifier (the lot ID) and wafers in the same lot can be given individual identifiers (the wafer ID). A wafer carries an identifying mark that typically is made up of the lot ID and a wafer ID. The identifier is commonly generated by a computer program. A typical wafer identifying mark has 11 characters and is about 13 millimeters in length.
An object of this invention is to provide a new an improved tool for producing this identifying mark.
In one technique of the prior art for forming a wafer identifying mark, a mark is etched into the silicon with a laser. Unfortunately, the laser may produce bumps of silicon that can cause micro scratches on the wafer after the wafer has gone the CMP process step (chemical and mechanical polishing).
The wafer marking tool of this invention uses a fiber optic bundle and an associated optical system to illuminate a photoresist in a pattern for an identifying character. Preferably, a switch able light source is provided for each fiber. These light sources are selectively switched on or off to form a pattern for a character. (In the specific description later, the letter xe2x80x9cTxe2x80x9d is used as an example.)
A layer of a resist is formed on the wafer and is exposed to the light pattern created by the marking tool. The resist is then developed and the wafer is etched in the pattern of the resist to form an arrangement of small holes that trace the character. The holes are about 100 microns in diameter and about 1 micron deep. The individual dots are too small to be seen, but the character is large enough to be read by operating personnel and by conventional machines for this purpose.
The character will ordinarily be etched during an etching step that is performed for wafer manufacture, and etching the character does not interfere with a simultaneous etch that takes place for manufacturing the wafer. The wafer resist is exposed separately for the identifying mark because the mark is unique to a single wafer.
During wafer manufacturing, a wafer etching step is commonly performed several times as a multi-layered structure is formed on the silicon substrate, and the wafer identifier mark becomes obscured by these process steps. Depositing metal on the wafer particularly obscures the mark. The etch step can be repeated without affecting the other steps of the manufacturing process.
Other objects and features of the invention will appear in the description of a preferred embodiment of the invention.